This invention relates to an apparatus for controlling the operation of an AC motor, and more particularly to an AC motor operation control apparatus having improved circuitry for regenerative braking.
AC motors, and induction motors in particular, are employed in a variety of industrial fields. The kinds of loads that can be driven by these motors are equally diverse. Some induction motors, for example, undergo rapid and frequent increases and decreases in speed, while in others there are positive and negative changes in load torque so as to permit the raising and lowering of an object in the manner of a hoist. Thus, there is a requirement that an AC motor functioning as a driving source be operated so as to generate a positive torque as well as a braking torque.
A method of controlling the operation of AC motors, which has recently come into fairly widespread use, employs a variable-voltage variable-frequency inverter. While no major projects are encountered in this method when operating the motor in the driving mode, one difficulty which does arise is the difficulty in dealing with the rotational energy possessed by the rotor of the motor when it is to be operated in the braking mode at the time of a reduction in speed. Two exemplary methods of dealing with this rotational energy have been adopted in the prior art. In one method the flow of current to the AC motor is interrupted at braking time to permit the motor to slow down naturally due to mechanical loss attributed to the load. In the other method the slip which arises at the time of the speed reduction is suitably controlled and is allowed to dissipate within the motor. The first method, however, requires too much time to achieve the speed reduction and has a very poor control response, while the second method causes the motor to overheat to such an extent that it cannot endure frequent increases and decreases in speed. Another method which can be mentioned is one in which the rotational energy of the rotor is dissipated by allowing a smoothing capacitor, inserted in the inverter circuitry mentioned above, to charge until the charged voltage exceeds a specified value, whereupon the capacitor is discharged through a braking resistor connected in parallel with the inverter circuit, thereby to dissipating the energy. However, this method is disadvantageous in that it may lead to destruction of the apparatus if the smoothing capacitor is allowed to charge to an excessively high value. This method is also expensive since the braking resistor increases in size and cost in accordance with the size of the machine to be driven by the motor. Moreover, the method is undesirable in terms of enhancing efficiency because of the fact that the braking energy is wasted in the form of thermal loss.
A regenerative braking method, as illustrated by the circuit diagram in FIG. 1, has been proposed in an effort to improve upon the foregoing arrangements.
The apparatus includes an AC motor 1 such as a three-phase induction motor; a rectifier 2 comprising diodes D.sub.1 through D.sub.6 for rectifying the U, V and W phases of the AC input power; a regenerative thyristor bridge circuit 3 comprising thyristors S.sub.1 through S.sub.6 ; a smoothing circuit 4 having a capacitor C.sub.1 ; a variable-voltage variable frequency inverter 5 comprising transistors TA.sub.1 through TA.sub.6 ; a rectifier 6 comprising diodes D.sub.1 ' through D.sub.6 '; and a step-up transformer 7 for boosting the power source voltage. To control the induction motor 1 with this conventional arrangement, for example, to reduce the motor speed, the command speed is lowered to control the voltage and frequency of the variable-voltage variable-frequency inverter 5. Thus, the synchronous speed, in conformance with the newly set frequency, becomes smaller than the motor speed, giving rise to a negative slip condition. Accordingly, the motor begins to run in the regenerative braking region, with the result that the voltage induced in the motor is rectified by the rectifier 6, thereby raising the voltage on the DC line side. The smoothing capacitor C.sub.1, in order for it to exhibit the smoothing function, is charged to a voltage which is 1.3 to 1.4 times the AC power source voltage even when the motor is operating in the normal driving mode. Nevertheless, when the induction motor is operated in the regenerative region, the smoothing capacitor C.sub.1 is charged to, and held at, an even higher voltage. For example, if the AC power source voltage is 200 volts, the voltage to which the capacitor C.sub.1 is charged is raised to approximately 290 volts. Under such a condition, commutation cannot take place and regenerative operation is impossible because the AC power source voltage is lower than the voltage on the side of the DC line even if the firing of the regenerative thyristor bridge circuit 3 comprising thyristors S.sub.1 through S.sub.6 is controlled. To avoid this inconvenience the step-up transformer 7 is inserted between the thyristor bridge 3 and the AC power source, and the circuitry is arranged in such a manner that there will be intervals in which the AC power source voltage is always higher than the voltage on the DC line side. This enables operation in the regenerative braking region while assuring commutation of the thyristors S.sub.1 through S.sub.6. However, the apparatus that employs this system is large and expensive due to the need for the large capacity step-up transformer 7.